Numerical and physical simulation of rapid microstructural evolution of gas atomised Ni superalloy powders
Abstract The rapid microstructural evolution of gas atomised Ni superalloy powder compacts over timescales of a few seconds was studied using a Gleeble 3500 thermomechanical simulator, finite element based numerical model and electron microscopy. The study found that the microstructural changes were governed by the characteristic temperatures of the alloy. At a temperature below the γ' solvus, the powders maintained dendritic structures. Above the γ' solvus temperature but in the solid-state, rapid grain spheroidisation and coarsening occurred, although the fine-scale microstructures were largely retained. Once the incipient melting temperature of the alloy was exceeded, microstructural change was rapid, and when the temperature was increased into the solid+liquid state, the powder compact partially melted and then re-solidified with no trace of the original structures, despite the fast timescales. The study reveals the relationship between short, severe thermal excursions and microstructural evolution in powder processed components, and gives guidance on the upper limit of temperature and time for powder-based processes if desirable fine-scale features of powders are to be preserved. Highlights Rapid microstructural changes of Ni powders were studied by physical and numerical simulations, and electron microscopy. γ' solvus temperature and incipient melting temperature were critical in controlling the microstructures of the Ni powders. Dendritic structures retained near γ' solvus temperature, but coarsened dramatically near incipient melting temperature. Powders melted and agglomerated near 50% liquid, re-solidified with coarser dendritic structures and micro-segregation. The findings provide general guidance for the temperature limits and time for retaining the fine microstructure of powders. Graphical abstract [DISPLAY OMISSION]
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